As it seems is well known, Syncthing has issues with some versions of Android which are granting read only access to folders on an SD card in a phone. This issue is not necessarily limited to Syncthing app. I have an Android 5 phone that a lot of apps are reporting the same type of issues with.
However some app designers claim they have been able to work around the issues on Android 4.4 or Android 5 and get write access to a folder which Syncthing apparently hasn’t been able to achieve so there appears to be an issue with programming in the app.
Many lower end Android phones now come with a stripped down version of the Android software called Go which reinstates the limitations on folder access per application and requires a specific coding to get access rights granted. I have a device running Android Go 8.1 and once again Syncthing’s app reports it can only get read access to regular folders on the SD card. But other third party apps (photo galleries for example) from the app store are popping up a little dialog that asks for the root folder on the SD card to be specified and then they have write access to any part of the card.
There have been comments about solutions that involve writing to folders on the SD Card starting with Android/data followed by the canonical name of the app and the biggest problem with these solutions is when you try to access the synced files with other apps and these apps are not able to see those folders.
In other words it seems the best solution for write access to a SD Card on some versions of android is to put in a feature request asking for the programmers to redesign the app according to the API model for access control.